1. Technical Field
The present invention relates to retaining walls and particularly to stackable precast concrete modular crib-type structures for creating retaining walls.
2. BACKGROUND INFORMATION
A solid block gravity retaining wall is probably the oldest known kind of retaining wall. This type of wall relies on the mass of the wall material and a configuration that normally decreases in thickness from base to top to resist the overturning force exerted by the pressure of earth behind it. Gravity retaining walls are usually made of stone, concrete block, or monolithic concrete poured at the site.
Another type of retaining wall is a cribbing wall constructed from interlocking timbers or precast concrete elements. U.S. Pat. No. 2,149,957 of DAWSON and U.S. Pat. No. 1,472,917 of LAIRD show examples of cribbing arrangements using precast concrete elements. The Dawson cribbing wall is elements: stretchers, headers, and anchors. The stretchers and headers are elongated straight elements having interengaging socket-and-lug ends that can be locked together by impaling rods. The stretchers form the face of the wall, and the headers extend perpendicularly to the face into the bank behind the wall. The anchors are U-shaped elements that interengage with the inner ends of the headers to create a hollow box-like structure, or crib. Each rigid element is cast separately, and the cribbing is assembled piece-by-piece at the site.
The LAIRD cribbing wall system also includes stretchers, headers, and anchors. In LAIRD, however, Y-shaped spread arms are cast integrally with a header member for anchorage purposes, thereby eliminating the necessity of tying the header member to another member to provide an anchor. According to LAIRD, the 45.degree. spread arms of the header member form an anchor with a maximum of efficiency, as the maximum amount of filling would have to be displaced before the header members would move. The forward end of each header carries two vertical pins that engage holes in adjacent ends of two stretchers laid end-to-end. These pins provide a positive connection between the header members and alternate rows of stretcher members. The stretcher members in adjacent rows are staggered, so that the stretcher members likewise are connected to alternate rows of header members, and the anchoring arms are so arranged that in straight wall construction the rear ends of alternate layers of headers bear on each other and may be pin-connected for accurate alignment.
One-piece precast cribbing or bin-type units also have been developed that can be stacked to make a vertical retaining wall. In most systems, individual units interlock with other units to form an integral structure of open bins which are subsequently backfilled with earth or gravel to add the necessary mass to the wall structure. These elements are of various shapes, but they typically have a flat face panel, a flat rear panel or a stretcher, and at least one web or connecting arm extending between the rear of the face panel and the rear panel or stretcher. U.S. Pat. No. 1,909,539 of Huntoon, U.S. Pat. No. 3,877,236 of O'Neill et al., and U.S. Pat. No. 4,372,091 of Brown et al., and French Patent Publication No. 2,409,351 provide examples of such units. In the O'Neill and Brown et al. units, the rear panel is usually the same length as the front panel or only slightly shorter. The rear panel of the Huntoon unit is significantly shorter than the front panel, and the module of French '351 has only solid wedge-shaped enlargements at the rear ends of two connecting arms.
The crib-type or bin-type systems, whether composed of individual face members, headers, and stretchers or of one-piece modular units, act essentially as gravity walls. That is, they resist lateral pressure and overturning moments by reason of the front-to-rear dimensions and the weights of the individual units, where the weight of a unit includes the weight of backfill material captured within the spaces between the front panels and rear panels or stretchers. As a result, the individual units, particularly the one-piece modular units, can become relatively large and correspondingly heavy.
Gravity walls of the types described above must be able to resist the pressure of backfill behind them. Since this pressure increases with depth of backfill, gravity walls typically are thicker at the base than at the top. As a practical matter, the front-to-rear dimension of precast concrete modules is limited by transport weight and size restrictions, so that modular gravity-type retaining walls have an ultimate height limitation.
An alternative to gravity-type retaining walls is the stabilized earth system as disclosed in U.S. Pat. No. 3,981,038 of VIDAL. In this system, a plurality of elongated reinforcing members are arranged in vertically spaced horizontal layers. Within each layer the members are spaced apart laterally, and some of the members extend at right angles to other members to form a lattice. Compacted earth fills the vertical array of horizontal lattices, and at least one elongated reinforcing member is attached to each of a number of precast concrete facing elements. The facing elements are in the form of panels which are provided with lugs extending from the rear faces, each lug being connected by bolts to one of the horizontal reinforcing strips.
Instead of relying on frictional resistance between soil and individual reinforcing strips, other soil reinforcing systems use welded wire mesh or grids. U.S. Pat. No. 4,324,508 of HILFIKER et al., U.S. Pat. No. 4,449,857 of DAVIS, U.S. Pat. Nos. 4,616,959 and 4,661,023 of HILFIKER, U.S. Pat. No. 4,728,227 of WILSON et al., and U.S. Pat. No. 4,824,293 of BROWN et al. disclose different arrangements for positively connecting metal grid or perforated polymer reinforcing mats to face panels of a retaining wall.
The stabilized earth systems are suited primarily to highway grade separation crossings, railway underpasses, and similar situations where minimal preliminary excavation is needed and there is ample space behind the wall for tamped backfill. The gravity type of retaining walls are normally employed where there is a steep cut bank with limited space behind the wall units for soil reinforcing members.